Alkanolamine-based carbon dioxide absorption solutions with reduced corrosivity

ABSTRACT

An aqueous carbon dioxide-absorbent solution comprising at least one alkanolamine and at least one quaternary ammonium salt is disclosed. A method of recovering carbon dioxide from a carbon dioxide-containing gaseous mixture and a method of reducing metal corrosion in an alkanolamine-based exhaust gas treating process are also disclosed.

The present invention relates to fossil fuel combustion, and, inparticular, to removal of carbon dioxide from fossil fuel combustion gasmixtures.

Carbon sequestration refers to the capture and storage of carbon thatwould otherwise be emitted to, or remain, in the atmosphere as carbondioxide (CO₂). Fossil fuels are the main source of world fuel and supplyover 85% of all primary energy, with nuclear, hydroelectric, solar,wind, and geothermal making up the rest. Although significant effortsand capital investments have been made by many nations to increaserenewable energy options and to foster conservation and efficiencyimprovements of fossil fuel utilization, climate change during thecoming decades will likely require significant effort and resources beexpended on carbon sequestration.

The potential impact of increasing concentrations of greenhouse gases inthe atmosphere will be a global concern well into the 21^(st) centuryand perhaps even beyond. There are three primary initiatives that theU.S. Department of Energy has undertaken to address the buildup ofgreenhouse gases. These include the improved efficiency of energyutilization, the adoption of low carbon fuels, and carbon sequestration.Carbon geosequestration is the process of not only capturing CO₂emissions that would otherwise be lost into the atmosphere, butpermanently storing them in geologic formations such as oil and gasreservoirs, abandoned coal seams, and deep ocean waters.

The primary sources of CO₂ are fossil-fueled power plants, industrialprocesses, and in the future, by-products of fuel decarbonizationplants. Power plants are the largest source and emit more than one-thirdof all CO₂ emissions worldwide. There are three primary routes forcapturing power plant CO₂ effluent: flue gas separation, oxy-fuelcombustion, and pre-combustion separation. While all three approacheshave been piloted, flue gas separation is currently the most commoncommercial method.

Flue gas separation and CO₂ capture are based on chemical absorptionwhereby CO₂ is reacted with an amine and captured as an adduct in theliquid phase. The most commonly used absorbent for CO₂ ismonoethanolamine (MEA), which carries with it significant problems suchas decomposition products requiring hazardous waste disposal and highcorrosivity to processing equipment.

Attempts have been made to address the problems associated with MEAtreatments. For example, U.S. Pat. No. 4,971,718 discloses the additionof antimony and N-methyldiethanolamine (MDEA) to an aqueous solution ofMEA. The combination of antimony and MDEA was supposed to inhibitcorrosion and retard solvent degradation. However, antimony compoundspresent potentially serious toxicity issues, and their use in largeindustrial applications could be problematic.

U.S. Pat. No. 4,477,419 discloses the use of copper salts in analkanolamine solution in conjunction with the use of an activated carbonor ion exchange resin. The copper salts combined with the use of anactivated carbon or ion exchange resin are supposed to lower corrosivityand/or degradative quality of the solvent. However, the use of carbonbeds or iron exchange resins adds further cost and complexity to the CO₂capture process.

In U.S. Pat. No. 4,596,849, a thiourea-amine-formaldehyde based polymeris identified as a corrosion inhibitor for CO₂ scrubbing using aqueousMEA. However, the process of U.S. Pat. No. 4,596,849 may producecarcinogens. In particular, the thiourea-amine-formaldehyde basedpolymer upon deterioration can release some level of formaldehyde, asuspected human carcinogen.

While research has been conducted using amines and amine derivatives asabsorption aids, there remains an opportunity for an innovativetechnology that displays improvement in reduction of corrosion in MEAsolutions, an increase in stability of the solvent, and/or lesscorrosion to the metal processing equipment found in towers and tanks.

It is, therefore, an object of the present invention to provide aqueousalkanolamine-based carbon dioxide absorbent solutions which exhibit lowcorrosivity to metallic equipment and do not contain or producecompounds which are toxic or harmful to the environment.

It has been found that certain quaternary ammonium salts are compatiblewith an aqueous alkanolamine solution and that the resultant combinationcan be used in sequestering carbon dioxide. Furthermore, it has beenfound that certain quaternary ammonium salts are not only fullycompatible with alkanolamines but provide significant reduction in thecorrosion rate of metal as measured by mm per year (mm·year⁻¹).

The present invention relates to an aqueous CO₂ absorbent compositioncomprising at least one alkanolamine having a minimum of 2 carbon atomsand a maximum of 6 carbon atoms, 1 or 2 hydroxyl groups, and a primary,secondary, or tertiary amino group, in an amount from 10% by weight to90% by weight, and, in an amount of from 0.03% by weight to 0.20% byweight, based on the total solution, at least one quaternary ammoniumsalt of the formula

wherein R¹ and R² are independently optionally aryl-substituted C₁₋₂₀alkyl groups, R³ and R⁴ are independently C₁₋₄ alkyl groups, X^(n−) isan anion selected from the group consisting of hydroxide, carbonate,bicarbonate, phosphates, phosphites, hypophosphite, nitrate, sulfates,borates, anions of saturated and unsaturated acyclic C₁₋₂₀monocarboxylic acids, anions of saturated and unsaturated C₂₋₂₀dicarboxylic acids, and anions of hydroxy-substituted carboxylic acids,and n denotes the appropriate number of negative charges of said anion.

Advantageously, the composition is essentially halogen free, and inparticular no halide anion is present in the at least one quaternaryammonium salt.

The invention also relates to a method of recovering CO₂ from aCO₂-containing gaseous mixture which includes contacting the gaseousmixture with an aqueous absorbent solution including at least onealkanolamine having a minimum of 2 carbon atoms and a maximum of 6carbon atoms, 1 or 2 hydroxyl groups, and a primary, secondary, ortertiary amino group, in an amount from 10% by weight to 90% by weight,based on the total solution, and at least one quaternary ammonium saltof the formula (I) above, wherein the at least one quaternary ammoniumsalt is present in an amount of from 0.03% by weight to 0.20% by weight,based on the total solution, when the method of recovering CO₂ from aCO₂-containing gaseous mixture takes place at a temperature greater than60° C., but less than 95° C.

In another embodiment, the method of recovering CO₂ from aCO₂-containing gaseous mixture takes place at a maximum temperature of60° C. and the at least one ammonium salt of the formula (I) is presentin an amount of from 0.01% by weight to 1.0% by weight.

Another embodiment of the invention relates to a method of reducingmetallic corrosion in an alkanolamine-based exhaust gas treating processincluding adding at least one quaternary ammonium salt of the formula(I) above to an aqueous alkanolamine solution, wherein the at least onequaternary ammonium salt of formula (I) is present in an amount of from0.03% by weight to 0.20% by weight and the gas-treating process isoperated at a temperature greater than 60° C., but less than 95° C.

In another embodiment, of the method of reducing corrosion in analkanolamine exhaust gas treating process, said process is operated at atemperature that does not exceed 60° C., and the quaternary ammoniumsalt is present in an amount of from 0.01% by weight to 1.0% by weight.

The at least one alkanolamine has a minimum of 2 carbon atoms and amaximum of 6 carbon atoms, 1 or 2 hydroxyl groups, and a primary,secondary, or tertiary amino group.

In a preferred embodiment, the at least one alkanolamine is selectedfrom the group consisting of monoethanolamine (MEA), diethanolamine,N-methyldiethanolamine, 2-amino-2-methyl-1-propanol,2-(ethylamino)-ethanol, 2-(methylamino)-ethanol, and mixtures thereof.The most preferred alkanolamine is monoethanolamine.

The at least one alkanolamine is present in a minimum amount of 10% byweight, preferably 20% by weight, and more preferably 30% by weight.

The at least one alkanolamine is present in a maximum amount of 90% byweight, preferably 80% by weight, and more preferably 70% by weight.

In a preferred embodiment of the invention, the at least one quaternaryammonium salt of the formula (I) is selected from the group consistingof didecyldimethylammonium bicarbonate/carbonate,didecyldimethylammonium phosphate, didecyldimethylammonium glycolate,didecyldimethylammonium hydroxide, or combinations thereof. The mostpreferred salt is didecyldimethylammonium bicarbonate/carbonate. Theexpression “bicarbonate/carbonate” is understood to mean bicarbonate,carbonate, or a mixture of bicarbonate and carbonate.

When the temperature involved in the methods described above does notexceed 60° C., the at least one quaternary ammonium salt of the formula(I) is present in a minimum amount of 0.01% by weight, preferably 0.05%by weight, more preferably 0.10% by weight, and most preferably 0.25% byweight. When the temperature involved in the methods described abovedoes not exceed 60° C., the at least one quaternary ammonium salt of theformula (I) is present in a maximum amount of 1.0% by weight, preferably0.75% by weight, more preferably 0.6% by weight, and most preferably0.5% by weight.

When the temperatures involved in the methods described above aregreater than 60° C., but less than 95° C., the at least one quaternaryammonium salt of the formula (I) is preferably present in a minimumamount of 0.03% by weight, more preferably 0.05% by weight, and mostpreferably 0.10% by weight. When the temperatures involved in themethods described above are greater than 60° C., but less than 95° C.,the at least one quaternary ammonium salt of the formula (I) ispreferably present in a maximum amount of 0.20% by weight, morepreferably 0.18% by weight, and most preferably 0.15% by weight.

It has been found that a formulation of at least one alkanolamine and atleast one quaternary ammonium salt of the formula (I) can be used in theprocess of sequestering carbon dioxide. The addition of at least onequaternary ammonium salt of the formula (I) to an alkanolamine solutionreduces the corrosion of the alkanolamine solution, increases thestability of the solvent, and leads to less corrosion to metallicprocessing equipment.

For a better understanding of the present invention, together with otherand further advantages, reference is made to the following detaileddescription, and its scope will be pointed out in the claims.

The present invention relates to a system which includes a compositionand method for absorbing carbon dioxide from exhaust gas (flue gas) fromfossil fuel combustion. The composition includes an aqueous solution ofan alkanolamine and at least one quaternary ammonium salt of formula

In the formula (I), R¹R²R³R⁴N⁺ represents the quaternary ammoniumcation. R¹ and R² are independently optionally aryl-substituted C₁₋₂₀alkyl groups. Accordingly, R¹ and R² are independently unsubstitutedC₁₋₂₀ alkyl groups or C₁₋₂₀ alkyl groups substituted with an aryl group.

C₁₋₂₀ alkyl groups are linear or branched alkyl groups having 1 to 20carbon atoms, including, but not limited to, methyl, ethyl, propyl,isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pentyl, isopentyl,hexyl, heptyl, octyl, nonyl, isononyl, decyl, dodecyl, tetradecyl,hexadecyl, octadecyl and icosyl. Aryl-substituted C₁₋₂₀ alkyl groups areany of the above groups bearing an aryl group, in particular phenyl, asa substituent. Preferred examples of aryl-substituted C₁₋₂₀ alkyl groupsare benzyl, phenylethyl and phenylpropyl.

In one preferred embodiment, R¹ is methyl. In another preferredembodiment, R² is benzyl or phenylethyl.

In another preferred embodiment, R¹ and R² are the same C₁₋₂₀ alkylgroups. More preferably, R¹ and R² are decyl groups. In an even morepreferred embodiment, R¹ and R² are n-decyl groups.

R³ and R⁴ are independently C₁₋₄ alkyl groups. The C₁₋₄ alkyl groups arelinear or branched and unsubstituted. Examples of C₁₋₄ alkyl groupsinclude, but are not limited to, methyl, ethyl, propyl, isopropyl, andbutyl.

R³ and R⁴ are preferably methyl groups.

The quaternary ammonium salt contains an anion to balance the charge ofthe quaternary ammonium cation. The anion may be singly, doubly, triplyor multiply charged. The anion is not a halide. Furthermore, halo anionsare not present in the at least one quaternary ammonium salt.

In particular, the expression “X^(n−)” represents an anion selected fromthe group consisting of hydroxide, carbonate, bicarbonate, phosphates,phosphites, hypophosphite, nitrate, sulfates, borates, anions ofsaturated and unsaturated acyclic C₁₋₂₀ monocarboxylic acids, anions ofsaturated and unsaturated C₂₋₂₀ dicarboxylic acids, and anions ofhydroxy-substituted carboxylic acids. The letter n denotes the valenceof the anion, i.e., the appropriate number of negative charges of theanion.

The term “phosphates” is to be understood as including both acid andneutral salts of phosphoric acid, namely, dihydrogenphosphates(X^(n−)═H₂PO₄ ⁻), monohydrogenphosphates (X^(n−)═HPO₄ ²⁻) and phosphates(X^(n−)═PO₄ ³⁻), as well as salts of oligo- and polyphosphoric acidssuch as diphosphates (pyrophosphates) and triphosphates.

Phosphites are salts of phosphorous acids containing the anions H₂PO₃ ⁻and/or HPO₃ ²⁻.

The term “sulfates” includes hydrogensulfates (X^(n−)═HSO₄ ⁻) andneutral sulfates (X^(n−)═SO₄ ²⁻) as well as disulfates (X^(n−)═S₂O₇ ²⁻)and related salts.

Borates are any salts containing anions derived from boric acid (H₃BO₃)and the various poly-boric acids.

Saturated and unsaturated acyclic C₁₋₂₀ monocarboxylic acids are inparticular alkanoic acids, such as formic, acetic, propionic, butyric,pentanoic, hecanoic, octanoic, decanoic, dodecanoic, tetradecanoic,hexadecanoic, octadecanoic and icosanoic acids, or alkenoic acids, suchas acrylic, methacrylic, oleic and linolic acid.

Saturated and unsaturated acyclic C₂₋₂₀ dicarboxylic acids are inparticular alkanedioic acids, such as oxalic, malonic, succinic,glutaric and adipic acid, or alkenedioic acids such as fumaric or maleicacid.

Hydroxy-substituted carboxylic acids are any carboxylic acids bearing atleast one hydroxy group in addition to the carboxylate group(s), such asglycolic, malic, citric or salicylic acid.

More preferred quaternary ammonium salts are quaternary ammoniumcarbonates, quaternary ammonium bicarbonates, quaternary ammoniumphosphates and quaternary ammonium glycolates. “Bicarbonate/carbonate”is defined as bicarbonate, carbonate, or mixtures thereof.

The most preferred quaternary ammonium cation is didecyldimethylammoniumand the most preferred quaternary ammonium salt isdidecyldimethylammonium bicarbonate/carbonate (DDABC).

The at least one quaternary ammonium salt includes combinations ormixtures of two or more quaternary ammonium salts of formula (I). Forexample, the at least one quaternary ammonium salt may be a mixture ofDDABC and didecyldimethylammonium hydroxide.

In the composition, the at least one quaternary ammonium salt is presentin a minimum amount of 0.03% by weight, preferably 0.05% by weight, andmore preferably 0.10% by weight. The quaternary ammonium salt is presentin a maximum amount of 0.20% by weight, preferably 0.18% by weight, andmore preferably 0.15% by weight in the composition.

The at least one alkanolamine may be any alkanolamine that can sequestercarbon dioxide. Effective alkanolamines are well-known in the art. SeeRobert J. Hook, “An Investigation of Some Sterically Hindered Amines asPotential Carbon Dioxide Scrubbing Compounds,” Ind. Eng. Chem. Res.1997, 365, 1779-1790 for examples of effective alkanolamines.

Suitably, the at least one alkanolamine has a minimum of 2 carbon atomsand a maximum of 6 carbon atoms, 1 or 2 hydroxyl groups, and a primary,secondary, or tertiary amino group. Examples of alkanolamines include,but are not limited to, monoethanolamine, diethanolamine,N-methyldiethanolamine, 2-amino-2-methyl-1-propanol,2-(ethylamino)-ethanol, and 2-(methylamino)-ethanol. The most preferredalkanolamine is monoethanolamine (MEA).

One alkanolamine or a combination or mixture of two or morealkanolamines may be present in the composition. For example, thecomposition may include a mixture of monoethanolamine andN-methyldiethanolamine.

The at least one alkanolamine is present in aqueous solution in aminimum amount of 10% by weight, preferably 20% by weight, and morepreferably 30% by weight. The at least one alkanolamine is present inaqueous solution in a maximum amount of 90% by weight, preferably 80% byweight, and more preferably 70% by weight.

In a particularly preferred embodiment, the composition includes 0.03%to 0.15% by weight DDABC in an aqueous solution containing 20% to 30% byweight of monoethanolamine.

In a preferred embodiment, the only active ingredients in thecomposition are the at least one alkanolamine and the at least onequaternary ammonium salt. For example, it is contemplated that otheractives such as peroxides are not included in the composition.

The composition of the invention may be used to sequester carbon dioxidefrom flue gas. The invention also relates to a method of recoveringcarbon dioxide from flue gas by contacting the flue gas with the aqueousabsorbent solution described above.

The invention further relates to a method for reducing metal corrosionin an alkanolamine flue gas treating process by adding at least onequaternary ammonium salt of formula (I) to an aqueous alkanolaminesolution.

The quantity of the at least one quaternary ammonium salt in thecomposition varies depending upon the temperatures at which the methodof recovering carbon dioxide from flue gas and the method of reducingmetal corrosion is operated. For example, if the method is operated at atemperature that does not exceed 60° C., the at least one quaternaryammonium salt of the formula (I) is suitably present in a minimum amountof 0.01% by weight, preferably 0.05% by weight, more preferably 0.10% byweight, and most preferably 0.25% by weight. When the temperatureinvolved in the method does not exceed 60° C., the at least onequaternary ammonium salt of the formula (I) is present in an amount ofno more than 1.0% by weight, preferably no more than 0.75% by weight,more preferably no more than 0.6% by weight, and most preferably no morethan 0.5% by weight.

When the temperatures involved in the methods described above aregreater than 60° C., but less than 95° C., the at least one quaternaryammonium salt of the formula (I) is preferably present in a minimumamount of 0.03% by weight, more preferably 0.05% by weight, and mostpreferably 0.10% by weight. When the temperatures involved in themethods described above are greater than 60° C., but less than 95° C.,the at least one quaternary ammonium salt of the formula (I) ispreferably present in a maximum amount of 0.20% by weight, morepreferably 0.18% by weight, and most preferably 0.15% by weight.

Herein, a list following the word “comprising” is inclusive oropen-ended, i.e., the list may or may not include additional unrecitedelements.

The instant invention contemplates embodiments in which each elementlisted under one group may be combined with each and every elementlisted under any other group. R¹ and R² are defined above asindependently representing optionally aryl-substituted C₁₋₂₀ alkylgroups. R³ and R⁴ are defined above as independently representing C₁₋₄alkyl groups. Each element of R¹ and R² (an aryl-substituted C₁₋₂₀ alkylgroup) can be combined with each and every element of R³ and R⁴ (a C₁₋₄alkyl group). For example, in one embodiment, R¹ is dodecyl, R² benzyl,R³ is methyl, and R⁴ is isopropyl. Alternatively, R¹ is 3-ethyltridecyl,R² is octyl, R³ is methyl, and R⁴ is ethyl.

With each group, it is specifically contemplated that any one of moremembers can be excluded. For example, if X^(n−) is defined as an anionselected from the group consisting of hydroxide, bicarbonate/carbonate,phosphates, phosphites, hypophosphite, nitrate, sulfates, borates,anions of saturated and unsaturated acyclic C₁₋₂₀ monocarboxylic acids,anions of saturated and unsaturated C₂₋₂₀ dicarboxylic acids, and anionsof hydroxy-substituted carboxylic acids, or combinations or mixturesthereof, then it is also contemplated that X^(n−) is defined ashydroxide, bicarbonate/carbonate, phosphates, nitrate, sulfates,borates, and anions of saturated and unsaturated acyclic C₁₋₂₀monocarboxylic acids.

However, the compounds used in this invention are limited to those thatare chemically feasible and stable. Therefore, a combination ofsubstituents or variables in the compounds described above ispermissible only if such a combination results in a stable or chemicallyfeasible compound. A stable compound or chemically feasible compound isone in which the chemical structure is not substantially altered whenkept at a temperature of 40° C. or less, in the absence of moisture orother chemically reactive conditions, for at least a week.

DESCRIPTION OF THE FIGURES

FIG. 1 shows the corrosion rate at 55° C. in mm per year (mm·year⁻¹)loss of metal. The corrosion rate was measured for three differentgroups of samples, each group including one sample containing a 30%aqueous solution of monoethanolamine (MEA) and one sample containing a70% aqueous solution of MEA. (All percentages are by weight.) Thehorizontal axis shows three groups: the control group in which nodidecyldimethylammonium salt was added to the MEA solutions, one groupin which 0.5% Carboshield® BTA was added to the MEA solutions, and onegroup in which 0.5% Carboshield® 1000 was added to the MEA solutions.

Carboshield® 1000 is 50% DDABC, 1.2% free amine (didecylmethylamine),3.3% methanol, 7.2% propylene glycol, and 38.3% water. Carboshield® BTAis mixture of 92.5% Carboshield® 1000 and 7.5% benzotriazole (BTA).

The corrosion rate for the 0.5% Carboshield® 1000 and MEA solutions wassignificantly lower than the MEA solutions by themselves and with 0.5%Carboshield® BTA. Therefore, adding 0.5% Carboshield® 1000 to a 30% or70% aqueous MEA solution at 55° C. can significantly reduce thecorrosion rate.

FIG. 2 shows the corrosion rate at 85° C. in mm per year (mm·year⁻¹)loss of metal. The corrosion rate was measured for four different groupsof samples, each group including one sample containing a 30% aqueoussolution of MEA and one sample containing a 70% aqueous solution of MEA.The horizontal axis shows four groups: the control group in which nodidecyldimethylammonium salt was added to the MEA solutions, one groupin which 0.5% Carboshield® BTA was added to the MEA solutions, one groupin which 0.5% Carboshield® 1000 was added to the MEA solutions, and onegroup in which 0.1% Carboshield® 1000 was added to the MEA solutions.

The corrosion rate for the 0.1% Carboshield® 1000 and MEA solutions wassignificantly lower than the MEA solutions by themselves and with 0.5%Carboshield® BTA or 0.5% Carboshield® 1000. Therefore, adding 0.1% ofCarboshield® 1000, i.e., DDABC to a 30% or 70% aqueous MEA solution at85° C. can significantly reduce the corrosion rate.

The present invention may be better understood by reference to thefollowing examples which illustrate the present invention and are notintended to limit the invention or its scope in any manner. Allpercentages are by weight, unless otherwise specified.

EXAMPLE 1

A solution was made containing 0.5% Carboshield® 1000 in a 30% aqueoussolution of monoethanolamine. Carboshield® 1000 is 50% DDABC, 1.2% freeamine (didecylmethylamine), 3.3% methanol, 7.2% propylene glycol, and38.3% water. Another solution was made containing 0.5% Carboshield® 1000in a 70% aqueous solution of monoethanolamine. The corrosion rates ofthese solutions are compared to control solutions of (1) 30%monoethanolamine or (2) 70% monoethanolamine, and solutions of (3) 0.5%Carboshield® BTA in 30% monoethanolamine or (4) 0.5% Carboshield® BTA in70% monoethanolamine. Carboshield® BTA is a mixture of 92.5%Carboshield® 1000 and 7.5% benzotriazole (BTA).

Cyclic potentiodynamic polarization (CPP) tests were performed with ASTM1018 carbon steel in monoethanolamine with and without added inhibitorsto investigate the corrosion behavior of carbon steel as well as therole of inhibitors. Two different concentrations of monoethanolamine(30% and 70%) and two inhibitors (Carboshield® BTA and Carboshield®1000) were used.

The CPP tests were performed at 55° C. and 85° C. under constant carbondioxide sparging. Prior to each test, the monoethanolamine solution wassparged with carbon dioxide for approximately 24 hours, the solutionthen was heated up to the desired temperature and the desiredconcentrations of inhibitors were added (no inhibitor was used in thecontrol tests). A platinized niobium wire loop was used as counterelectrode and a tungsten/tungsten trioxide wire was used as thereference electrode. The CPP curve was obtained by scanning thepotential at a rate of 0.17 mV/s from −0.1V to 1V vs. W/WO₃ or until thecurrent reached 1 mA/cm², whichever occurred first.

Upon completion of the test, the sample was removed from the testingsolution and was inspected for the extent of corrosion. To provide adirect comparison of inhibitor performance, the corrosion rate wascalculated from the corrosion current extracted from the CPP curvesusing the equation below:

${CR} = {K_{l}\frac{i_{corr}}{\rho}{EW}}$

CR=corrosion rate (mm per year)K₁=conversion constant, 0.003272 mm·year⁻¹·μA⁻¹·g·cm⁻³i_(corr)=observed corrosion current (μA)ρ=density (g/cm³)EW=equivalent weight of carbon steel, 27

The results of the corrosion rate comparisons are shown in FIGS. 1 and2.

1. An aqueous carbon dioxide-absorbent solution comprising: from 10% byweight to 90% by weight, based on the total solution, of at least onealkanolamine having from 2 to 6 carbon atoms, 1 or 2 hydroxyl groups,and a primary, secondary, or tertiary amino group; and (ii) from 0.03%by weight to 0.20% by weight, based on the total solution, of at leastone quaternary ammonium salt of the formula

wherein: R¹ and R² are independently optionally aryl-substituted C₁₋₂₀alkyl groups; R³ and R⁴ are independently C₁₋₄ alkyl groups; X^(n−) isan anion selected from the group consisting of hydroxide, bicarbonate,carbonate, phosphates, phosphites, hypophosphite, nitrate, sulfates,borates, anions of saturated and unsaturated acyclic C₁₋₂₀monocarboxylic acids, anions of saturated and unsaturated C₂₋₂₀dicarboxylic acids and anions of hydroxy-substituted carboxylic acids,and n denotes the appropriate number of negative charges of said anion.2. The composition of claim 1, wherein X^(n−) is selected from the groupconsisting of bicarbonate, carbonate, phosphates, glycolate andhydroxide.
 3. The composition of claim 1, wherein the at least onealkanolamine is selected from the group consisting of monoethanolamine,diethanolamine, N-methyldiethanolamine, 2-amino-2-methyl-1-propanol,2-(ethylamino)-ethanol, 2-(methylamino)-ethanol, and mixtures thereof.4. The composition of claim 1, wherein the at least one alkanolamine ispresent in an amount of at least 20% by weight.
 5. The composition ofclaim 4, wherein the at least one alkanolamine is present in an amountof at least 30% by weight.
 6. The composition of claim 1, wherein the atleast one alkanolamine is present in an amount of no more than 80% byweight.
 7. The composition of claim 6, wherein the at least onealkanolamine is present in an amount of no more than 70% by weight. 8.The composition of claim 1, wherein R¹ and R² are independently C₈₋₁₂alkyl groups.
 9. The composition of claim 8, wherein R¹ and R² are decylgroups.
 10. The composition of claim 1, wherein R³ and R⁴ are methylgroups.
 11. The composition of claim 10, wherein the at least onequaternary ammonium salt of formula (I) is selected from the groupconsisting of didecyldimethylammonium bicarbonate,didecyldimethylammonium carbonate, didecyldimethylammonium phosphate,didecyldimethylammonium glycolate, didecyldimethylammonium hydroxide,and mixtures thereof.
 12. The composition of claim 1, wherein the atleast one quaternary ammonium salt of formula (I) is present in anamount of at least 0.05% by weight.
 13. The composition of claim 12,wherein the at least one quaternary ammonium salt of formula (I) ispresent in an amount of at least 0.10% by weight.
 14. The composition ofclaim 1, wherein the at least one quaternary ammonium salt of formula(I) is present in an amount of no more than 0.18% by weight.
 15. Thecomposition of claim 14, wherein the at least one quaternary ammoniumsalt of formula (I) is present in an amount of no more than 0.15% byweight.
 16. The composition of claim 1, wherein the alkanolamine ismonoethanol-amine and the at least one ammonium salt of formula (I) isdidecyldimethylammonium bicarbonate and/or didecyldimethylammoniumcarbonate.
 17. The composition of claim 1, wherein said at least onealkanolamine and said at least one ammonium salt are the only activeingredients in the composition.
 18. A method of recovering carbondioxide from a carbon dioxide-containing gaseous mixture, comprisingcontacting said carbon dioxide-containing gaseous mixture with anaqueous absorbent solution comprising: from 10% to 90% by weight, basedon the total solution, of at least one alkanolamine having from 2 to 6carbon atoms, 1 or 2 hydroxyl groups, and a primary, secondary, ortertiary amino; and (ii) from 0.01% to 1.0% by weight, based on thetotal solution, of at least one quaternary ammonium salt of the formula

wherein: R¹ and R² are independently optionally aryl-substituted C₁₋₂₀alkyl groups; R³ and R⁴ are independently C₁₋₄ alkyl groups; X^(n−) isan anion selected from the group consisting of hydroxide, bicarbonate,carbonate, phosphates, phosphites, hypophosphite, nitrate, sulfates,borates, anions of saturated and unsaturated acyclic C₁₋₂₀monocarboxylic acids, anions of saturated and unsaturated C₂₋₂₀dicarboxylic acids, and anions of hydroxy-substituted carboxylic acids;and n denotes the appropriate number of negative charges of said anion.19. The method of claim 18, wherein the at least one quaternary ammoniumsalt of formula (I) is present in an amount of 0.03% by weight to 0.20%by weight.
 20. The method of claim 18, wherein the carbondioxide-containing gaseous mixture is contacted with the aqueousabsorbent solution at a temperature of no more than 60° C.
 21. Themethod of claim 19, wherein the carbon dioxide-containing gaseousmixture is contacted with the aqueous absorbent solution at atemperature of between 60° C. and 95° C.
 22. A method of reducing metalcorrosion in an alkanolamine-based exhaust gas treating process, saidmethod comprising the step of adding to an aqueous solution containingfrom 10% to 90% by weight, based on the total solution, of at least onealkanolamine having from 2 to 6 carbon atoms, 1 or 2 hydroxyl groups,and a primary, secondary, or tertiary amino group, from about 0.01% byweight to 1.0% by weight, based on the total solution, of at least oneammonium salt of the formula

wherein: R¹ and R² are independently optionally aryl-substituted C₁₋₂₀alkyl groups; R³ and R⁴ are independently C₁₋₄ alkyl groups; X^(n−) isan anion selected from the group consisting of hydroxide, bicarbonate,carbonate, phosphates, phosphites, hypophosphite, nitrate, sulfates,borates, anions of saturated and unsaturated acyclic C₁₋₂₀monocarboxylic acids, anions of saturated and unsaturated C₂₋₂₀dicarboxylic acids, and anions of hydroxy-substituted carboxylic acids,and n denotes the appropriate number of negative charges of said anion.23. The method of claim 22, wherein the at least one quaternary ammoniumsalt of formula (I) is present in an amount of 0.03% by weight to 0.20%by weight.
 24. The method of claim 22, wherein the gas treating processis operated at a temperature of no more than 60° C.
 25. The method ofclaim 24, wherein the gas treating process is operated at a temperaturebetween 60° C. and 95° C.
 26. The method of claim 18, wherein X^(n−) isselected from the group consisting of bicarbonate, carbonate,phosphates, glycolate and hydroxide.
 27. The method of claim 18, whereinthe at least one alkanolamine is selected from the group consisting ofmonoethanolamine, diethanolamine, N-methyldiethanol-amine,2-amino-2-methyl-1-propanol, 2-(ethylamino)-ethanol,2-(methylamino)-ethanol, and mixtures thereof.
 28. The method of claim18, wherein the at least one alkanolamine is present in an amount of atleast 20% by weight.
 29. The method of claim 28, wherein the at leastone alkanolamine is present in an amount of at least 30% by weight. 30.The method of claim 18, wherein the at least one alkanolamine is presentin an amount of no more than 80% by weight.
 31. The method of claim 30,wherein the at least one alkanolamine is present in an amount of no morethan 70% by weight.
 32. The method of claim 18, wherein R¹ and R² areindependently C₈₋₁₂ alkyl groups.
 33. The method of claim 32, wherein R¹and R² are decyl groups.
 34. The method of claim 18, wherein R³ and R⁴are methyl groups.
 35. The method of claim 18, wherein the at least onequaternary ammonium salt of formula (I) is selected from the groupconsisting of didecyldimethylammonium bicarbonate,didecyldimethylammonium carbonate, didecyldimethylammonium phosphates,didecyldimethylammonium glycolate, didecyldimethylammonium hydroxide,and mixtures thereof.
 36. The method of claim 18, wherein the at leastone quaternary ammonium salt of formula (I) is present in an amount ofat least 0.05% by weight.
 37. The method of claim 36, wherein the atleast one quaternary ammonium salt of formula (I) is present in anamount of at least 0.10% by weight.
 38. The method of claim 37, whereinthe at least one quaternary ammonium salt of formula (I) is present inan amount of at least 0.25% by weight.
 39. The method of claim 18,wherein the at least one quaternary ammonium salt of formula (I) ispresent in an amount of no more than 0.75% by weight.
 40. The method ofclaim 39, wherein the at least one quaternary ammonium salt of formula(I) is present in an amount of no more than 0.60% by weight.
 41. Themethod of claim 40, wherein the at least one quaternary ammonium salt offormula (I) is present in an amount of no more than 0.50% by weight. 42.The method of claim 18, wherein the at least one quaternary ammoniumsalt of formula (I) is present in an amount of no more than 0.18% byweight.
 43. The method of claim 42, wherein the at least one quaternaryammonium salt of formula (I) is present in an amount of no more than0.15% by weight.
 44. The method of claim 18, wherein the at least onealkanolamine is mono-ethanolamine and the quaternary ammonium salt offormula (I) is didecyldimethyl-ammonium bicarbonate and/ordidecyldimethylammonium carbonate.
 45. The method of claim 22, whereinX^(n−) is selected from the group consisting of bicarbonate, carbonate,phosphates, glycolate and hydroxide.
 46. The method of claim 22, whereinthe at least one alkanolamine is selected from the group consisting ofmonoethanolamine, diethanolamine, N-methyldiethanol-amine,2-amino-2-methyl-1-propanol, 2-(ethylamino)-ethanol,2-(methylamino)-ethanol, and mixtures thereof.
 47. The method of claim22, wherein the at least one alkanolamine is present in an amount of atleast 20% by weight.
 48. The method of claim 22, wherein the at leastone alkanolamine is present in an amount of no more than 80% by weight.49. The method of claim 22, wherein R¹ and R² are independently C₈₋₁₂alkyl groups.
 50. The method of claim 22, wherein R³ and R⁴ are methylgroups.
 51. The method of claim 22, wherein the at least one quaternaryammonium salt of formula (I) is selected from the group consisting ofdidecyldimethylammonium bicarbonate, didecyldimethylammonium carbonate,didecyldimethylammonium phosphates, didecyldimethylammonium glycolate,didecyldimethylammonium hydroxide, and mixtures thereof.
 52. The methodof claim 22, wherein the at least one quaternary ammonium salt offormula (I) is present in an amount of at least 0.05% by weight.
 53. Themethod of claim 22, wherein the at least one quaternary ammonium salt offormula (I) is present in an amount of no more than 0.75% by weight. 54.The method of claim 22, wherein the at least one quaternary ammoniumsalt of formula (I) is present in an amount of no more than 0.18% byweight.
 55. The method of claim 22, wherein the at least onealkanolamine is monoethanolamine and the quaternary ammonium salt offormula (I) is didecyldimethylammonium bicarbonate and/ordidecyldimethylammonium carbonate.